1. Field of the Invention
The present invention relates to a gradient-index lens which is used in optical communication, display devices, image-reading devices such as scanners, information-storage devices such as optical discs, etc.
2. Related Art
Lenses are utilized in various optical fields of optical communication, display devices, etc. Of those, gradient-index lenses are differentiated from other ordinary homogeneous lenses in that their characteristics are defined only by the external shapes of the lenses.
Gradient-index lenses include, for example, those having a concentric circular refractive index profile 16 that runs from the center axis 70 of a cylindrical substrate 11 toward the outer periphery thereof, as in FIG. 13A; those having a semispherical refractive index profile 26 that runs from the surface of a planar substrate 21 toward the inside thereof, as in FIG. 13B; and those of a spherical or non-spherical lens 32 with a refractive index profile 36 introduced in the direction of the optical axis thereof, as in FIG. 13C.
In the following description, the lenses as in FIG. 13A are referred to as rod lenses; those as in FIG. 13B are as planar lenses; and those as in FIG. 13C are as axial gradient-index lenses. These will be all referred to as a generic term of gradient-index lenses. The rod lenses are cylindrical and their lens faces may be flattened, and therefore, their maintenance and positioning are easy. In addition, since their shape is well compatible with the shape of optical fibers, the rod lenses are widely used especially in the field of optical communication. The planar lenses are suitable to fabrication of microlens arrays with many microlenses aligned, and they are suitable to optical systems for processing light in parallel.
These lenses are all required to have a reduced-refractive index on their lens races. There are two purposes of reducing the refractive index on the lens faces, which are mentioned below.
One is to reduce as much as possible the loss to be caused by the lens insertion (insertion loss); and another is to reduce as much as possible the negative influence of the light reflected on the lens face to return to the light-emitting side (referred to as “reflected return light”), and this is especially important in the field of optical communication.
For reducing the surface reflection of gradient-index lenses, generally employed is a method or coating the lens face with a single-layered or multi-layered, thin dielectric film. This is referred to as an antireflection (AR) film, and is used not only in gradient-index lenses but also in any ordinary homogeneous curved lenses and other various parts and articles.
According to the method, the insertion loss is reduced and the reflected return light is also reduced. However, when the reflected return light must be reduced more, often employed is a method of polishing the lens face 17 of a gradient-index lens 13 in the direction oblique to the optical axis 72 to thereby make the reflected light 76 from the lens face 17 shifted from the running direction of the incident light 14, as in FIG. 14.
The method may be combined with the AR film 19, as in FIG. 14. In a planar lens 23 as in FIG. 16, not the lens face but the back of the substrate 27 is polished in the oblique direction, whereby the lens face is inclined relative to the optical axis and the reflected return light can be thereby reduced.
The above-mentioned methods have been already applied to commercial products, and gradient-index lenses having an extremely reduced reflectance on the lens faces are widely used.
However, the above-mentioned methods have some problems mentioned below.
AR films shall have a different reflectance, depending on the refractive index of the material to be coated with them. Literally, the gradient-index lenses of FIGS. 13A, 13B and 13C all have a different refractive index in the center and the periphery of the lenses. Accordingly, for making the entire lens face have good antireflection capability, the AR film to be formed on the lenses must be so designed that its antireflection capability may vary in accordance with the refractive index profile of the lenses varying from the center to the periphery of the lenses.
Such a high-performance AR film generally has an increased number of constitutive layers, and will take a long time for forming the layers, and, as a result, the cost of the lenses inevitably increases. When an inexpensive AR film having a small number of layers is used, it still undergoes surface reflection in some degree, owing to the refractive index profile of the lens substrate as so mentioned hereinabove, and it does not satisfy the recent high-level requirement for antireflection.
In addition, the AR film still has other problems mentioned below.
Another embodiment of gradient-index lenses is known as in FIG. 15, in which two lenses 13a and 13b are made to face each other via an optical functional part 42 such as an optical filter sandwiched between them, and the light running from the optical fiber 43a is, after having received the action of optical functional part 42 that acts thereon, led into the other optical fiber 43b. For the structure of this embodiment, the optical fibers 43a, 43b, the lenses 13a, 13b, and the optical functional part 42 must be bonded to each other via an adhesive 30 to assemble them into the lens structure.
The capability of the AR film 19 varies depending on the refractive index of the substrate, as so mentioned hereinabove. In a case where the outer face of the AR film 19 is kept in contact with anything other than air (in this embodiment, this is kept in contact with the adhesive 30), as in FIG. 15, the property of the AR film s further varied depending on the refractive index of the medium to which the film is contacted. For example, the refractive indices of lens substrates and adhesives individually vary in a different manner depending on the ambient temperature around them, and, as a result, the antireflection capability of the AR films adjacent to them shall vary in accordance with their variation. Further, if their refractive indices have varied owing to the deterioration of the lens substrates and the adhesives, then the antireflection capability of the AR films adjacent to them will also vary. The antireflection capability change in the AR films is also a serious problem in view of the recent high-level requirement for antireflection of lenses.
In the related art technology, a method of polishing the lens face of a gradient-index lens in an oblique direction is employed, as so mentioned hereinabove, but the oblique polishing in the method has the following problem.
Polishing micro parts in an oblique direction requires a complicated operation. In addition, since the lens face is polished obliquely, the mechanical center axis of the lens will be inevitably shifted from the optical center axis thereof for attaining good light concentration through the lens. Therefore, when the thus-processed microlens is built in an optical module, its constitution shall be complicated.
The constitutive parts may be aligned almost linearly, as in FIG. 15. Also in this structure, however, the optical axis center and the mechanical center of the lens are shifted from each other in some degree, and therefore, the structure is more readily influenced by the lens aberration. This will result in the increased requirement for higher registration accuracy in parts assembly, and therefore in the increase in the production costs.